Pipe handling devices, commonly called snubbing or pipe push/pull machines, are used in drilling operations to push pipe into and pull pipe out of a well bore. Pipe push/pull machines include a pair of pipe gripping devices. The pipe gripping devices are mounted on an assembly that maintains the devices in vertical alignment and, while the bottom pipe gripping device is held stationary, moves the upper pipe gripping device vertically toward and away from the lower pipe gripping device. The pipe gripping devices each carry slip members that can be alternately engaged with and released from a pipe being pushed into or pulled from the well.
Pipe gripping devices are known which include a plurality of radially moveable carrier blocks on which slip members are mounted. The slip members can be shifted radially inwardly by the movement of the carrier blocks until teeth on the inner surfaces of the slips engage against the outer surface of a pipe passing through the center of the pipe gripping device. The slip members normally are centered with respect to the horizontal axis of the carrier blocks and are moveably retained in slots in the carrier blocks. Each slot has a pair of wedged surfaces that are co-operable with companion wedged surfaces on the slip member in a manner such that any vertical movement of the slip over the wedged surfaces of the carrier block will drive the slip radially inwardly or outwardly relative to the center of the pipe gripping device. The wedged surfaces are arranged in opposition such that the slip drives radially inward when pushing or pulling pipes with the pipe gripping device.
The carrier blocks are moved by use of spiral gear plates which engage teeth formed in the upper and lower surfaces of the carrier blocks.
However, previous devices have no safety systems to prevent release of pipe which is subject to substantial longitudinal force, such force being due to string weight or high well head pressure.
In addition, when the carrier blocks of previous devices are withdraw radially away from pipe gripping position, they tend to lock against the outer wall of the housing. This locking is caused by the carrier blocks loading against and, thereby, locking against the outer wall. The inertia in the movement of the carrier blocks causes the force of their impact against the housing to be greater than the force which can be exerted by the motor to move the carrier blocks radially inward. The only solution to such jamming has been to increase the size of the drive means. This solution is undesirable as it increases both the cost, as well as the size and the portability of the pipe gripping devices.
Increased use of underbalanced drilling (UBD) technology, where most wells are designed and operated to maintain less than 100 psi in the annulus at surface, has created the need for pipe handling devices able to operate in both the pipe heavy and pipe light conditions and which are able to withstand the forces placed upon them without jamming or binding. Increased use has also required that safety concerns be addressed while the industry has demanded smaller, more portable devices.